Question about safe water temperature?

Today I was fishing in the Boise river, I enter the water about up to my knees so I could cast a bit further into the river. The newspaper said the water temperature was about 58 degrees. I stood in the chilly water for about 10 minutes then it felt like the water suddenly got warm. Not paying any attention to it I continued to fish, after about an hour I got out of the water. Once on dry land i almost collapsed because my legs were completely numb, after taking several steps my Achilles Tendons on both legs cramped up really bad. Just wondering what causes this (besides the cold.... obviously) and is it safe to do this, I just dont wanna have to get anything amputated and I am planning on going first thing tomorrow morning, again. The air temp was 96 degrees F. I was not wearing waders, just bare skin. THANKS! :)